1. Technical Field
The present invention is related generally to integrated circuits. More specifically, the present invention is directed toward voltage regulator circuits.
2. Description of Related Art
Integrated circuits continue their trend to higher transistor densities and smaller feature sizes. As the technology in different devices forces different supply requirements, various low voltage standards are propagating. While early xe2x80x9clogic circuitsxe2x80x9d used 5V, today""s devices require 5V, 3.3V, 2.8V, 2.5V, 2.0V, 1.8V, 1.5V, 1.2V, 0.9V and others. Each type of xe2x80x9clogic circuitxe2x80x9d requires a voltage supply that consistently supplies an appropriate stable voltage with little or no voltage fluctuations in order to prevent erroneous output from the xe2x80x9clogic circuitxe2x80x9d. Various types of voltage regulators are provided in the prior art for this function.
One type of prior art single output voltage regulator is illustrated in FIG. 1. FIG. 1 depicts a schematic diagram illustrating a prior art technique in which a shunt regulator (U2) is combined with a voltage divider to provide regulation of a single output power supply. In this case, the output voltage Vout is divided down by resistors R1 and R2 and compared to the internal reference in the shunt regulator U2. The error voltage that results from this comparison is typically fed back through an optocoupler 104 (an optocoupler, also known as an optoisolater, is a coupling device in which a light emitting diode, energized by the input signal, is optically coupled to a photodetectors such as a light-sensitive output diode, transistor, or silicon controlled rectifier) to adjust the duty ratio of the switching converter, thereby regulating the output voltage Vout. For this structure, the output voltage Vout can be adjusted by modifying the current in the voltage divider 100 through resistor R3 (henceforth xe2x80x9cdivider trimxe2x80x9d). If current is sunk from the output adjust terminal 102, the regulation voltage Vout is increased; if current is sourced into the adjust terminal 102, the output voltage Vout is decreased.
FIG. 2 depicts a schematic diagram illustrating the other prevalent prior art technique for single output regulation. Here, an independent two-terminal reference is used (instead of a shunt regulator as in FIG. 1) with an opamp U1A to build an error amplifier. While the divider trim can also be used here (by tying R3 to the junction of R1 and R2), the displayed circuit employs xe2x80x9creference trimxe2x80x9d R3. Here, a current applied through reference trim R3 can increase or decrease the value of the reference being delivered to the amplifier U1A at the non-inverting input terminal 206. This change in reference voltage results in a change in output voltage Vout1. Notice that the polarity of the adjustment will be opposite that of the divider trim; a current sink from the adjust terminal 202 lowers the output voltage Vout1, and a current source into the adjust terminal 202 increases output voltage Vout1.
The reference trim has several advantages. It offers a consistent percentage change in output voltage Vout1 for a given adjust current, regardless of the output divider ratio. It also allows for very wide trim range without impacting the gain of the feedback control loop. The trim input can also be heavily filtered for noise immunity, without impacting the loop speed or stability.
Although the systems described above are useful for systems requiring one voltage rail, they are not as appropriate for the mixed low voltage systems requiring various voltages that have become commonplace. The packaging density and thermal demands have likewise continued to grow with each new generation of product. As a result, there is a need for power converters with high density, high efficiency, and multiple outputs, to energize these systems.
Multiple output switching power modules in standard footprints are frequently used in communications systems to fill the market needs. Multiple vendor sources for these supplies are often required, driving some common specifications. Many multiple output supplies rely on cross regulation, achieved through scaled winding ratios, to deliver multiple outputs with only one feedback regulation loop. Cross-regulated units suffer several drawbacks: poor load regulation, crosstalk, poor resolution in setting output voltages, complicated magnetic designs, and no flexibility for independent output adjustment.
For cross-regulated units, regulation is frequently achieved through a TL431 type shunt regulator which senses a weighted version of the multiple outputs. This regulator develops an error signal, which is used to modulate the duty cycle of the power switch. Output voltage adjustment is necessarily a tracking output adjustment, as the two cross-regulated outputs are always proportionally scaled. A 10% adjustment will adjust both outputs by 10%. As the inexpensive TL431 does not normally offer a pinned-out, adjustable reference, the only means for adjusting the set point is to modify the current in the weighted output voltage divider.
An example of a shunt regulator is provided in FIG. 3, which depicts a schematic diagram illustrating a prior art extension of the usage of the reference trim to a multiple output supply. Here, the feedback divider is the weighted average of two output voltages Vout1 and Vout2, generated by the summing node at the junctions of R1 and R2. For cross-regulated supplies, this technique is desirable since it can help compensate for poor load regulation when the two output loads are imbalanced. For this circuit, one can determine that the DC trim pin 302 current is a function of all the divider resistors (R1-R4), the reference value of the shunt regulator, R5, Vout1 and Vout2, and the impedance on the trim pin 302. Output voltage Vout1 and Vout2 adjustment through the single trim pin 302 is necessarily a tracking output adjustment, as the two cross-regulated outputs Vout1 and Vout2 are always proportionally scaled. For example, a 10% adjustment will adjust both outputs Vout1 and Vout2 by 10%.
Switching post regulators (or other methods of independent regulation) offer superior flexibility, regulation and set point selection. The outputs feature independent feedback loops, so the regulation of the outputs is not necessarily linked. To take advantage of this flexibility, post-regulated multiple outputs often offer two adjustable references and two output adjustment pins, so that the outputs may be adjusted independently. The outputs are compared to their references with separate regulation amplifiers, providing excellent regulation.
Consequently, the post regulator output adjustment circuits are often not compatible with those of cross-regulated units, making it difficult to drop them into applications using cross-regulated modules. The xe2x80x9cdivider trimxe2x80x9d method results in a different trim impedance, and sinking current from the trim pin results in an increased output voltage. The xe2x80x9creference trimxe2x80x9d method lowers the output when sinking current from the trim pin.
To offer a product with even broader application, a new approach is needed: a method to offer either tracking output adjustment or independent adjustment in a post-regulated circuit, while maintaining the regulation advantages of the post regulator.
The present invention provides a simple method to offer either independent output adjustment or tracking output voltages in a post regulated converter. A new resistor divider and reference structure is introduced, which offers simplicity and low parts count in achieving the two different output voltage adjustment functions. In one embodiment, the voltage regulator includes a plurality of output voltage pins, a plurality of independent trim pins, a tracking trim adjust pin, a plurality of voltage error pins, a reference voltage supply; and a plurality of operational amplifiers. The plurality of output voltage pins are coupled to a first input and an output of the operational amplifiers. The reference voltage supply is coupled a second input of each of the operational amplifiers. The tracking trim adjust pin is coupled to the first input of each of the operational amplifiers. The independent trim pins are coupled to the second input of the operational amplifiers and the voltage error pins are coupled to the output of the operational amplifiers.